The invention relates to an energy storage device, a system having an energy storage device and a method for driving an energy storage device, in particular in a battery direct converter circuit for simultaneous power supply to an electric machine and generation of a further voltage level for a DC voltage system.
The trend is that in the future electronic systems which combine new energy storage technologies with electrical drive technology will be used increasingly both in stationary applications, such as wind power installations or solar installations, and in vehicles, such as hybrid or electric vehicles.
The feed of polyphase current to an electric machine is generally provided by a converter in the form of a pulse-controlled inverter. For this purpose, a DC voltage provided by a DC voltage intermediate circuit can be converted into a polyphase AC voltage, for example a three-phase AC voltage. The DC voltage intermediate circuit is in this case fed by a line of battery modules connected in series. In order to be able to meet the requirements for power and energy provided for a respective application, often a plurality of battery modules are connected in series in a traction battery.
The series circuit comprising a plurality of battery modules is associated with the problem that the entire line fails when a single battery module fails. Such a failure of the energy supply line can result in failure of the entire system. Furthermore, temporarily or permanently occurring power reductions of an individual battery module can result in power reductions in the entire energy supply line.
Document U.S. Pat. No. 5,642,275 A1 describes a battery system with an integrated inverter function. Systems of this type are known under the name multilevel cascaded inverter or else battery direct inverter (BDI). Such systems comprise DC sources in a plurality of energy storage module lines, which can be connected directly to an electric machine or an electrical power supply system. In this case, single-phase or polyphase supply voltages can be generated. The energy storage module lines in this case have a plurality of energy storage modules which are connected in series, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit, which makes it possible to interrupt the respective energy storage module line or to bypass the respectively associated at least one battery cell or to switch the respectively associated at least one battery cell into the respective energy storage module line, depending on control signals. By suitable driving of the coupling units, for example with the aid of pulse width modulation, suitable phase signals for controlling the phase output voltage can also be provided, with the result that there is no need for a separate pulse-controlled inverter. The pulse-controlled inverter required for controlling the phase output voltage is therefore integrated in the BDI, so to speak.
BDIs generally have a higher degree of efficiency and higher degree of failsafety in comparison with conventional systems. The failsafety is ensured, inter alia, by virtue of the fact that defective, failed or not fully effective battery cells can be disconnected from the energy supply lines by suitable bypass driving of the coupling units. The phase output voltage of an energy storage module line can be varied, and in particular set stepwise, by correspondingly driving the coupling units. The stepping of the output voltage in this case results from the voltage of an individual energy storage module, with the maximum possible phase output voltage being determined the sum of the voltages of all energy storage modules of an energy storage module line.
Documents DE 10 2010 027 857 A1 and DE 10 2010 027 861 A1, for example, disclose battery direct inverters with a plurality of battery module lines, which can be connected directly to an electric machine.
There is no constant DC voltage available at the output of BDIs since the energy storage cells are divided between different energy storage modules and the coupling directions thereof need to be driven in a targeted manner for generating a voltage level.
There is therefore a need for an energy storage device and a method for operating said energy storage device, with which two different voltage levels, including in particular a DC voltage level, can be provided simultaneously.